In a conventional connection structure between a coaxial connector and a multilayer board, the signal line pattern of the transmission line (hereinafter, referred to as ‘electroconductive pattern’) of a high-frequency multilayer board (hereinafter, referred to as ‘multilayer board’) mounted in an electroconductive casing and the core wire of a coaxial connector have been directly electrically connected by use of suitable brazing material (see, for instance, JP-A-2001-177311).
The operation will now be described as below.
A high frequency signal input from the coaxial connector passes the connection between the coaxial connector and the multilayer board, and propagates through the transmission line formed on the multilayer board; and also in the propagating line in the direction opposite thereto, a high frequency signal that has propagated through the transmission line passes the connection between the coaxial connector and the multilayer board, and propagates toward the coaxial connector.
In the above operation, in the connecting portion between the coaxial connector and the multilayer board, the spacing between the grounding face on the casing and the core wire of the coaxial connector becomes large because the multilayer board has its thickness, to thereby cause the impedance of the transmission line in this area to be greatly disordered, and induce the transmission line to be dielectric. Here, there is a trend toward larger degree of the dielectric property thereof with increasing thickness of the multilayer board. In addition, in the multilayer board, because it is difficult to form the transmission line pattern in the uppermost layer of this board and the ground pattern in the layer located immediately under the layer closely to the end face of the board, a large pattern margin forms between the end face of the board and the end of the ground pattern. For this reason, in the vicinity of the end face of the multilayer board, it is impossible to form the ground pattern constituting the transmission line. Therefore, there arises a problem that the impedance of the transmission line is greatly disordered also in this area to thereby induce the transmission line to be dielectric.
On this account, in the conventional connection structure between the coaxial connector and the multilayer board, it is arranged that the coaxial connector is equipped with a capacitive coaxial line in order to cancel the above-mentioned dielectric property of the transmission line, and thereby an impedance match is schemed to ensure its performance capabilities.
The conventional connection structure between the coaxial connector and the multilayer board has been arranged as mentioned above. As a result, around the frequencies designed such that the impedance match is provided, the capacitive coaxial line provided on the coaxial connector and the dielectric transmission line provided in the connection between the coaxial connector and the multilayer board mutually cancel the capacitive property and the dielectric property to thus render the matching. This reduces the disorder of impedance, thereby enabling the obtainment of a certain level of return-loss characteristic. However, at frequencies away from the designed frequencies, the match cannot be provided, and the disorder of impedance in the connection between the coaxial connector and the multilayer board increases, thereby making it impossible to obtain the low return-loss characteristic indispensable to the high-frequency circuit. From these factors, in the conventional connection structure between the coaxial connector and the multilayer board, it is very difficult to obtain favorable return-loss characteristics over a wide frequency band. As a result, there is a problem that countermeasures to development of broadband in communication equipments seen in recent years cannot be implemented.
The present invention has been accomplished to solve the above-mentioned problem. An object of the present invention is to provide a connection structure between a coaxial connector and a multilayer board that can greatly reduce the disorder of impedance due to the electrical connecting portion between the coaxial connector and the board, and that can obtain an excellent return-loss characteristic over a broad band.